Method and apparatus for measuring the amplification ratio of electric amplifiers



Nov. 10, 1931. I r. LAURENT 1,831,744

METHOD AND APPARATUS FOR MEASURING THE AMPLIFICATION RATIO OF ELECTRICAMPLIFIERS Filed March 30, 1929 Elm:

0.00 I a Y'- j Kimrerd' T 59 any other suita Patented Nov. 40, 1931UNITED STATES? mrrnon AND armm'ros ron' MEASURING TEN -onion a TOBBERNLAURENT, or srocxnomu; swEnnN, AssIeNon-r'ro TELEFONAKTIEBOLAGE'I. L. M.EBIGSSON, or srooxnonm, swEnE A COMPANY or SWEDEN THE AmrLIrIcnrroNnA'rro or ELECTBIG mrmrrnns v 7 Application filed March so, 1929, SerialNo A plication filed in Sweden March 28, 1928. I a

I The present invention relates to a method and an apparatus fordetermining or measuring the amplification ratio of electr c am- '5plifiers. The invention has for its ob ect to simplify such measurementsas far as possible and to provide an apparatus of simple and reliableconstruction for the purpose. specified. More particularly, theinvention has for itsobject to make it possible to per form suchmeasurements without using auxiliary voltages, oscillators or otherexpenslve apparatus whereby amplification measurements may be aneconomic possibility also in.

According to the invention the output side of the amplifier is connectedto the input side thereof over a net-work or the like the damp. i ing ofwhich is so adjusted as to cause the. amplifier to produceself-oscillations, in which case the amplification is at least of such avalue as to cover the damping of the oscillating system. If the dampingis so ad- 'u'sted that the condition of'self-oscillations Just startsthe value of the damping of the net-work may be'used as a measure of theamplification ratio.

=The invention will be more closely "described with reference totheaccompanying drawings in which Figure 1. is a circuit dia-: gram showinga measuring apparatus 50$ cording to an embodiment of the inventionconnected to an electric amplifier. Figure 2 showsadiagram. v s i Y" Y IReferring to Fig. 1, A is the electric amplifier, the output terminals 2of which are back-con led by means of the measuring apparatus s owninside thedashed lines B to the input terminals 1. The output terminals4 of the measuring apparatus are connected to the input terminals 1 ofthe amplifier and the output terminals 2 of the amplifier to'the in-,

put terminals 3 of the measuring apparatus. According to the shownembodiment the put terminals 3 an adjustable. distortionless artificialdamping device D which by way ofexample may be composed of resistancescombined in H-sha e as shown in Figure 1 or in lile manner. The outputterrepeater circuits of the most simple kind...

measuring apparatus contains next to the in- 1 351,318,-and in Swedenmarches, 19 2s.

minals of the artificial clamping device are I connected to the primarywinding of atransformer T provided with three windings. The

primary and secondary windings are disposed symmetrically in relation,to an intermediate shield 5 which is earthed at E. A

telephonereceiver H is connected to the third of the measuring apparatusa resonance shunt 1s inserted whlch shunt comprises an induc-.

tance L and avariable condenser G connected in parallel thereto. v I Theapparatusmay be usedin the follow 1ng manner: The resonance shunt L, Gis tuned to the frequency at which the amplification ratiois desired tobe measured. The

damping of the adjustable. artificial damping oleViceD is varied to thepoint of silence 1. etc the limit of setting the amplifier intoself-oscillations which limit may be observed the telephonereceiver H.The amplifier willlthen as a rule'oscillate at said resonance frequency.If the resonance impedance of the resonance shunt is sufliciently highincomparison with the characteristic impedance of the artificial damping Dand the input. impedance, of the amplifier, the additional damping inthemeasuring apparatus introduced by the transformer T the selfmductance Land the capacity C is negligible. Provided the phase displacement in;the amplifier i. e. the phase angle between the out- 1 put and the inputvoltages lies within certain limits the absolute value of theamplification is practically equalto the absolute'value of the damping.I j p Figure 2 shows the-differenoet between the real and the measuredamplification as a function of the phase displacement aof the amplifier.This difference mayv be expressed through the formula I y l 1 7 I As maybe seen from said-formula comparatively great phase displacements maybeallowed'before said dilferenceis of any practical importance;

the resonance shunt L, Chas a phase distorting effect of such acharacter that it auto-'" Ego When measuring the amplification ratio inamplifiers in which filters are included considerable phasedisplacements may occur. The phase displacement, however, variesconsiderably with the frequency and generally, as a consequence, thereare nearly always frequencies at which the phase displacement isinsignificant and the present measuring apparatus maythen be used/ Saidfrequencies are characterizedby that the measured amplification has amaximum equal to the real amplification at those frequencies; Guided bythis fact, said frequencies may thus be found. Experience has further"shown that the resonance shunt L, C at frequencies near to the naturalfrequency of matically compensates the phase displacement intheamplifier. Consequently tliedifference' between real and measuredamplification becomes substantially less than the i abovementioned'theoretical value. 'As, fur

ther, the damping of the resonance shunt is constant at the saidfrequencies, the measured" result will be easy to correct for saiddamping, if required.

' The shown arrangement of the transformer T has for its object toshieldoff unsymmetric currents and voltages which may occur undercertain conditions, for instance if the branches of the back-couplingnet-work arenot symmetrically arranged, and which would-produce anoscillating state substan? tially independent of the resonance shunt.The shield '5 then forms a kind of short cir- 'cuit for occasionalcharging currents "which otherwise would pass across the intermedi-' a"completely symmetric arrangement, be re placed by other arrangements orIn certain ate space between the transformer windings; The transformermay, however, especially in cases omitted altogether."

The measuring apparatus may :in certain casesbe simplified, by way ofexample, if it is a question of amplification measurements inanamplifier in which the amplification is adjustable by means of forinstance a poten-- tuned to a certain, desired frequency. How ever, itis'thenmor'e .diificult to establish'the" desired symmetry in theback-coupling de vice. Different kinds'of filters may'also be usedinstead of the oscillatory circuit but tiometer. No variable parts arethen necessary in the measuring apparatus. Such asimplified measuringapparatusmay also be used in case it is desired to ascertainthat theamplification ratio of the amplifier reaches a certain minimum value.

' Instead of disposing the oscillatory circuit" L, C as a shunt betweenthe two branchesof' the back coupling device as in the shown embodiment,self-inductances and capacities may be connected into circuit in thebranches proper in suclramanner that they can be andtheinput side ofthe'amplifierf such filters are as a rule unnecessarily complioated and theyintroduce also a damping which is not negligible. On the other hand suchdamping means need not necessarily introduce ainyerror in the measuringresult if at the position of silence.

In addition to the above mentioned-ad? "vantages of the measuringapparatus i. eits simplicity and -chea 'oness, the absence of delicateparts and the' fa'ct that no auxiliary 'voltagesor auxiliary currentsources are re quired when applying the apparatus, it may I". "bementioned that measurements with the apparatus are very easy to performand that the measuring result is vfree from 'sub]ective measuringerrors.

As the point at which thedamping isjust? balanced by the amplificationis determined by the starting of self-oscillations "erroneousmeasurements on account-of overloading the amplifier never need to befeared.

I claim: 7 I a '1. A method for determiningthe amplification ratio of'anelectric amplifier which consistsin'connecting the o'utput'side of anamplifier to the input side'thereof over a back-coupling, choosing thedamping of the system formed-so as to-he equal to a predetermined valuecorresponding to 'apcertain average amplification ratio,and ascertainingby means of a suitable indicatorwhether the system is in a'stat e'ofself-oscillation in which 'ca'se'thfe'amplification' is at least.ofgsuch a value as to 'coverit-heg damping of the oscillat- 'ingSySteni.- i

2. A method for determining the amplification ratio of an electric:amplifienwhich consists in connecting-the 'output 'sideiof anamplifier'to"the inputside' thereof over a back coupling,varying thedamping of the system. formed so as'to cause the amplifier toproduceselfsoscillaticns, and determining the value ofthe dampingfor-which thecondition of self-oscillations starts.) V

An apparatus for determining-the. amplification ratio of anelectricamplifier comipri sing in combination a net-work with damp in'g'properties and an' indicator connected with saidfnetwork fordetectingoscillations therein, said network being adaptedto serve a back-couplingbetween the 'output'sid 3 ion 4. An apparatus for measuring theamplification rat1o of an electric amplifier, com prising in combinationa network, means for varying the value of the damping of said net-;

5 work and an indicator connected with said i network for detectingoscillations therein, said network being adapted to serve as aback-coupling between the'output side and the input side of the amlifier.

5. An apparatus or determining the amplification ratio of an electricamplifier comprising in combination a network, a tunable V oscillatorycircuit connected therewith, an indicator connected with the network fordetecting oscillations therein, said network being adapted to serve as aback-coupling between the output side and the input side of theamplifier.

6. An apparatus as claimed in claim 5,'

characterized by that the oscillatory circuit consists of an inductanceshunted between the branches of the network and a condenser connected inparallel with said inductance.

In testimony whereof I aflix my signature.

TORBERN LAURENT.

